A fundamental property of differentiated cells is that they utilize a unique combination of gene products, some that are ubiquitously expressed and others that are cell type specific. We have studied the promoter regions of several B lymphocyte specific genes to help understand the mechanisms restricting their expression to B cells. For example, the CD19 gene is expressed shortly after commitment to the B cell lineage. Analysis of its promoter identified several key regulatory elements including an element that binds the B cell specific activator protein, BSAP. The development of antibodies against BSAP allowed us to identify an unusual cytoplasmic form of BSAP that has a smaller molecular mass than the nuclear BSAP. This unusual form of BSAP has only been found in progenitor B cells. Activating these cells increases CD19 expression and shifts the cytoplasmic form of BSAP to the nuclear form. The mechanisms that account for this shift are under study. We have also found that BSAP is dramatically over-expressed in certain high grade B cell lymphomas. To evaluate the mechanisms regulating BSAP gene transcription in normal B cells and B cell lymphomas we have isolated the BSAP promoter region and studies are in progress to determine the important regulatory regions. The CD20 gene is also B lymphocyte lineage specific, but expressed later in development than CD19. Analysis of its promoter identified several key cis-elements that control its stage- and cell-specific expression. One of them is a composite binding site for the transcription factors PU.1 and PIP. Together PU.1, PIP, the macrophage and B lymphocyte specific PU.1 and the lymphocyte specific PIP, account for much of the lineage and stage specific expression of CD20. Another approach to analyze the role of transcription factors in lineage commitment and B cell development is to create mice deficient in a specific transcription factor ("knock-out" mice). HB9 is a human homeobox gene expressed at high levels in a subset of B lymphocytes. We have targeted the murine HB9 gene in embryonic stem cells to generate HB9 "knock out" mice. Analysis of the mice is in progress. Overall, these studies are leading to a better understanding of the complex interplay between transcription factors and their target sequences in generating lineage specific gene expression.